The mid-70's found me at the NASA Ames Research Center on the Search for Extraterrestrial Intelligence. In 1979 I begun a three year stint working for U.S. Senator Spark Matsunaga in DC. In the office was Harvey Meyerson, who ghost-wrote The Mars Project: Journeys Beyond the Cold War, authored by Senator Matsunaga (hey, politicians don't have time to actually write a book). Harv for the Senator proposed working with the Soviet Union on a mission to Mars. We shared the same suite in the Senate so had ample discussions.
Moon Village, where the U.S., European Union, Russia, China, Japan, India and other nations cooperate to share information and long-term benefits of such an effort. The implications for peace would be priceless.
In searching for info, I found another book Harvey published: Launchpad for the 21st Century: Yearbook of the International Space Year. He has also written other books you can find in Amazon.com.
If these simple trips can cost that much, how can SpaceX (Elon Musk) charge "only" half a million dollars for sending you to Mars? He plans to do this in around a decade. Not sure if there are any options to return. How can such credible organizations be so disparate in cost?